KDE

Yeah, I agree that there could probably be a way to "close" Activities, which doesn't do the session management, so explicitly just throws the windows onto another Activity (or maybe prompts you when there's still windows on that Activity), without having to outright delete that Activity.

Deleting an Activity is relatively disruptive, since you may have files linked to it or nicely setup wallpapers and such. And there are a number of places where Activities show up, where it can be annoying to have Activities showing up that you're not currently using.

I can imagine them being open to that suggestion, if you articulate it well.
From what I saw, they did make a lot of changes to remove the start/stop functionality, but most of it was session handling code. So, it might not be too additional much trouble to add a way to close Activities instead.

As a wise Nate Graham once said: The most reliable way to find out whether people use a feature (and how they use it) is to remove it. The second-most reliable way is to announce its removal.
Well, you did miss the announcement, so it probably felt a bit rude to you, but yeah, you should still consider this the start of a conversation. They're not hellbent on removing this feature.